But data shows that children who take exams at 15 or under are less likely to do well.

Just over half of early entrants gained A* to C grades in English last year compared with more than two-thirds of exams taken by 16-year-olds, it emerged.

The DfE insist that the latest reforms will put pressure on schools to only enter pupils for GCSEs “when they are confident the student has the best opportunity to succeed”.

Michael Gove, the Education Secretary, criticised schools that seek to “cheat” to improve their league table position.

“The school is in effect gaming the system by not thinking what is in the best interests of the student but using the student as a means of gathering points so that school itself can look better," he said. "That is, in a word, cheating.

“When a small minority cheat, the system is corrupted for the others. That has to stop.”

Under new rules, schools will be required to include pupils’ first attempt at a GCSE in their official rankings – shunning any subsequent exams taken in the same subject.

The change takes immediate effect for all subjects covered by the English Baccalaureate – a league table measure that marks out pupils with good grades in English, maths, science, foreign languages and history or geography. It covers the 2013/14 academic year and will affect next summer’s results.

The reforms will cover other subjects such as art, music, drama, design and technology, computing and religious education a year later.

A Department for Education spokesman said: "Schools should not be entering children for exams before they are ready.

“It is not good for pupils to be put in early in a school’s hope they can ‘bank’ a C – it is far better that children study the subject fully and take it when they are ready.

“Some schools, having ‘banked’ a C grade in maths a year early, then enter pupils for other exams to game the league table system, instead of continuing with maths to 16. This is bad for the pupils and is another strong reason to make the change we are announcing.

"We have already announced we are scrapping modules and moving to end-of-course exams. Both will help stop schools 'gaming' the system in this way.

“Changing the system so that results in first exams count will ensure there is no benefit to schools who simply want to put children in for GCSEs before they are ready.”

However, head teachers' leaders criticised the change, claiming it was unfair on pupils.

Brian Lightman, general secretary of the Association of Teachers and Lecturers, said: “We have repeatedly warned about the damaging effects of piecemeal changes to the qualifications system.

"It is grossly unfair to make changes like this when courses are already underway. This is adding pressure and stress to students in the most important year of their education.

"Whatever the rights and wrongs of early entry, students, teachers, parents and employers just do not know where they are in the context of constant tinkering with examinations.”